PM / OPP: Move away from RCU locking

The RCU locking isn't well suited for the OPP core. The RCU locking fits
better for reader heavy stuff, while the OPP core have at max one or two
readers only at a time.

Over that, it was getting very confusing the way RCU locking was used
with the OPP core. The individual OPPs are mostly well handled, i.e. for
an update a new structure was created and then that replaced the older
one. But the OPP tables were updated directly all the time from various
parts of the core. Though they were mostly used from within RCU locked
region, they didn't had much to do with RCU and were governed by the
mutex instead.

And that mixed with the 'opp_table_lock' has made the core even more
confusing.

Now that we are already managing the OPPs and the OPP tables with kernel
reference infrastructure, we can get rid of RCU locking completely and
simplify the code a lot.

Remove all RCU references from code and comments.

Acquire opp_table->lock while parsing the list of OPPs though.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>